Glutaraldehyde and 1,2-benzisothiazolin-3-one are both individually known to exhibit desirable biocidal activity. Thus, Payne et al (U.S. Pat. No. 4,188,376) discloses that 1,2-benzisothiazolin-3-one is known to be a very effective biocide, particularly for the protection of aqueous media against infection by microorganisms. Similarly Clifford et al (U.S. Pat. No. 4,539,071) exemplify the biocidal efficacy of glutaraldehyde alone.
While the blending of glutaraldehyde with other biocides, such as 2-methyl-4-isothiazolin-3-one and 5-chloro-2-methyl-4-isthiazolin-3-one has been accomplished with desirable results in the past (see, e.g., U.S. Pat. No. 4,539,071 to Clifford et al), similar results have not been exemplified for blends of glutaraldehyde with 1,2-benzisothiazolin-3-one. It is believed that this stems in large part from the perceived incompatibility of glutaraldehyde with 1,2-benzisothiazolin-3-one.
Thus, it is well accepted that glutaraldehyde should be formulated into acidic compositions, as this compound polymerizes rapidly in alkaline environments See, e.g., K.-E. Rasmunen et al, "Glutaraldehyde. The Influence of pH, Temperature and Buffering on the Polymerization Rate", Histochemistry, Vol. 38, pp. 19-26 (1979): S. Thomas et al, "Temperature-Induced Changes in the Sporicidal Activity and Chemical Properties of Glutaraldehyde", Applied Microbiology, Vol. 28, No. 3, pp. 331-335 (September, 1974). While Japanese Patent Publication 63-112532 does disclose weakly alkaline aqueous glutaraldehyde solutions, such result is only achieved at the expense of biocidal activity.
Conversely, due to the low solubility of 1,2-benzisothiazolin-3-one in water under acidic conditions, it has become well accepted that this compound must be formulated under alkaline conditions to produce concentrated aqueous solutions. Thus, for example, U.K. Patents Numbers 1,171,253 and 1,330,531 both show aqueous formulations of 1,2-benzisothiazolin-3-one which are stabilized by the addition of an appropriate amine or mixture of amines
Consequently, it is completely unexpected that a concentrated aqueous solution of 1,2-benzisothiazolin-3-one and glutaraldehyde could be prepared, much less that such combination, when diluted by adding it to an aqueous-based industrial product, would exhibit synergistic biocidal activity.